Routes of Excretion
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Transcript Routes of Excretion
all drugs not in gaseous state need to use
fluid routes of excretion
◦ fluid routes include -sweat, tears, saliva, mucous,
urine, bile, human milk
◦ amount of drug excreted in each of these fluids is
in direct proportion to amount of fluid excreted
SO…….
numerous functions –
◦ filters out metabolic products
numerous functions –
main function – maintain correct balance
between water and salt in body fluids
◦ filters out metabolic products
◦ blood continuously flowing through kidneys
factors that influence a substance not being
resorbed
not lipid soluble
ionized
dialysis –
absorption, distribution and excretion do
not occur independently
first pass metabolism
blood
brain
1.
Body weight - smaller size
•
concentration of drug based on body fluid
2.
Sex differences
3.
Age
4.
Interspecies differences
rabbits – belladonna (deadly nightshade)
5.
Intraspieces differences
6.
Disease states
7.
Nutrition
8.
Biorhythm
half-life - time takes for the blood
concentration to fall to half its initial value
after a single dose
½ life tells us critical information about how
long the action of a drug will last
How long would it take for a
drug to reach 12.5% remaining
in blood if its ½ life is 2 hours?
How long would it take for a
drug
to reach 12.5% remaining in
blood if its ½ life is 100 hours?
Provides a good indication of the time
necessary to reach steady state after a dosage
regime has been initiated (6X)
drug elimination = drug availability
usually try and maintain steady state
concentration in therapeutic window
So if a drug had a 3 hour ½ life – how long
would it take to reach steady state?
Therapeutic drug monitoring - branch of
clinical chemistry that specializes in the
measurement of medication levels in blood.
Its main focus is on drugs with a narrow
therapeutic range,
- need to reach threshold plasma
concentration at the receptor site to initiate
and maintain a pharmacological response.
◦ assume that plasma represents good indicator of
local site
TDM is actually indirect
How is TDM determined?
What happens if?
◦ Plasma levels are too high –
◦ Plasma levels are too low –
Focus on levels rather than dose
info on a range of doses of drug
dose usually presented on horizontal axis
(log concentration)
size of effect or percentage affected usually
on vertical axis
the intensity or magnitude of the response
in a single person
the % of people who exhibit a characteristic
effect at a given dosage
potency - amount of drug required to elicit
a response
slope of the line tells you about how much
difference in drug is needed for small
effects relative to larger effect
Efficacy - maximum effect obtainable
- peak of the DRC indicates the maximum effect
Variability and slope –
individual differences in drug response
Different DRC depending upon measure of
interest
ED 50 - The dose of a drug that produces
the desired effect in 50% of the population
LD 50 –
TI = Therapeutic Index – measure of safety
LD 50/ED 50
hypothetical drug that can be used as a sedative –
this is tested in mice –
** dose cannot guarantee 100% sleeping and no deaths
Caution in interpreting DRC
Often see a bell-shaped curve in response to drug
antagonist - one drug diminishes the effect
of another
agonist – one drug is additive to the effect of
another
Pharmacodynamics
◦ drugs produce their effects by binding to and
interacting with receptors
What is a receptor?
◦ usually a protein on the surface or in the cell
each NT binds to its own receptors
◦ there can be multiple receptor subtypes
each NT binds to its own receptors
◦ there can be multiple receptor subtypes
useful for understanding drugs that work on
the specific neurotransmitters
1. ionotropic postsynaptic receptors
quick action and over quickly
Ion channel - close
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Ion channel - open
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Ligand-gated channels
Neurotransmitter
receptor
Ca2+ -activated
K+ channel
Cyclic nucleotide
gated channel
Na+
Na+
Glu
cAMP
Ca2+
K+
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cAMP
K+
cGMP
K+
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2. G-protein coupled receptors
◦ (metabotropic)
◦ 2nd messenger systems
◦ more than 50 G protein coupled receptors have
been identified
◦ control many cellular processes
3. carrier proteins (transporter)
◦ presynaptic transporters – transport NT back into
presyn ending
4. enzymes –
◦ what is an enzyme?
◦ breakdown NT -
1. the drug binds to the same location that
the endogenous NT occupies
results in similar effects as NT – agonist
2. binds to a site near the binding site for
the NT
◦ facilitates NT binding
◦ allosteric effect
◦ modulatory effects
3. binding to a receptor site normally
occupied by the NT but not activating
receptor and blocking NT
◦ antagonist
certain drugs may be more potent than the nt
expected results – due to the principal
actions of the drugs
less expected –
no drug is completely selective
definition?
types of tolerance
◦ metabolic tolerance – enzyme induction
◦ pharmacodynamic tolerance –
chemical see-saw
drug
brain response
The brain wants to rebalance the activity
definition?
types of tolerance
◦ metabolic tolerance – enzyme induction
◦ pharmacodynamic tolerance –
◦ behavioral tolerance